As permanent magnets exhibiting a high energy product of 25 MGOe or greater, so-called rare-earth magnets (R—Fe—B magnets, where R is a rare-earth element such as neodymium as in the following) have been developed. As such rare-earth magnets, for example, Patent Documents 1 and 2 disclose one formed by sintering and one formed by rapid cooling, respectively.
Though the rare-earth magnets exhibit a high energy product, their corrosion resistance is relatively low since they contain a rare-earth element and iron which are relatively easy to oxidize as main ingredients.
For ameliorating the corrosion resistance of such rare-earth magnets, it has been proposed to form a protective layer. Among them, Patent Document 3 proposes to form a protective layer by heating a rare-earth magnet at 200 to 500° C. in an oxidizing atmosphere.    Patent Document 1: Japanese Patent Application Laid-Open No. SHO 59-46008    Patent Document 2: Japanese Patent Application Laid-Open No. SHO 60-9852    Patent Document 3: Japanese Patent Application Laid-Open No. HEI 5-226129